As Is well known, valve timing has a significant effect on engine performance and the optimum setting varies with engine operating conditions. To optimise performance under different operating conditions, it is necessary to be able to vary the valve timing. Complex systems have been proposed that vary the duration of valve events, this being equivalent to using a cam with a different profile, while other systems only vary the phase of a camshaft acting on one set of valves relative to the engine crankshaft and/or relative to a second camshaft acting on the remaining valves.
Various phase change mechanisms have been proposed in the past but they have suffered from various problems. Some, though feasible, have been costly to implement while other have developed excessive friction or not proved to be reliable. Furthermore, many could not be fitted as a modification to existing engines as they required much of the valve train and cylinder head to be redesigned.
The Applicants' earlier EP-A-0 733 154 discloses a valve operating mechanism comprising a hollow shaft, a sleeve journalled on the hollow shaft and fast in rotation with a cam, a coupling yoke connected by a first pivot pin to the hollow shaft and by a second pivot pin to the sleeve and means for moving the yoke radially to effect a phase change between the hollow shaft and the sleeve. The means for moving the yoke radially comprise an actuating rod slidably received in the hollow shaft, a cam surface on the actuating rod and a plunger passing through a generally radial bore in the hollow sleeve to cause the yoke to move radially in response to axial movement of the actuating rod.
The above valve operating mechanism is only one example in which a phase change is brought about by axial movement of an actuating rod relative to the camshaft. Other phase change mechanisms that use an actuating rod movable axially relative to the camshaft are also known. The present invention is particularly concerned with a hydraulic actuator for displacing the actuating rod of such a phase l5 change mechanism.
It has already been proposed to mount a hydraulic jack on the drive pulley or sprocket of the camshaft and to connect the actuating rod of the piston of the hydraulic jack. The most common prior art proposal for supplying oil to the hydraulic jack employs drillings in the camshaft. In such a case, however, the length of the drillings and the restrictions placed on their diameter, make it difficult to ensure an adequate supply of oil to the hydraulic jack to allow the phase of the camshaft to be adjusted rapidly.